Kahn Fotuali'i powered over and Biggar added a drop goal before Isaacs appeared to put the game to bed with his try.
But Treviso eventually broke through when Zanni powered over to set up a tense finale. And with their tails up, Treviso rampaged up field with Giulio Toniolatti and Valerio Bernabo both prominent in the build-up to Pratichetti's match-winning score at the death.
After the defeat Ospreys coach Jonathan Humphreys, said: "It is disappointing to lose and let it slip like that but sometimes that's the way it goes. We ran out of petrol. We gave our all but just couldn't hang on.
"I'm not going to criticise the young players, they have been great through the competition for us. We didn't get the result but they stepped up and delivered again."